1. Field of the Invention
The present invention relates to photolithography used in the manufacturing of semiconductor devices and the like. More particularly, the present invention relates to a baking unit for baking a layer of photoresist during a photolithography process.
2. Description of the Related Art
In the manufacture of semiconductor devices, photolithography is used to transfer the image of a pattern, such as a circuit pattern, to a wafer. Photolithography typically entails a coating process of coating the wafer with photoresist, an exposure process of aligning the wafer with a mask bearing the pattern, and directing light such as ultraviolet light through the mask and onto the layer of photoresist, and a developing process of developing the exposed layer of photoresist to remove select (exposed or non-exposed) portions of the photoresist and thereby pattern the layer of photoresist. The photolithography process may further include a baking process of baking the photoresist before and after the exposure process to harden the photoresist, an HMDS (hexamethyl disilazane) treatment process of treating the wafer with HMDS to increase the adhesiveness between the photoresist and the wafer, and a cooling process of cooling the wafer.
A typical photolithography apparatus, therefore, includes units to coat the wafer with photoresist, expose the photoresist, bake the photoresist and develop the exposed photoresist, respectively. Highly-advanced semiconductor devices can only be manufactured using highly advanced photolithography apparatus which comprise large numbers of such individual processing units. In this respect, a recently developed photolithography apparatus has an inline system of a large number of coating, exposure and developing units which enables the multi-processing of wafers. Such an inline system is disclosed in U.S. Pat. No. 6,654,668.
Wafers are transferred between the units in the inline system by a transfer robot. Although the transfer robot may operate at a high speed with the aim of maximizing the yield of the photolithography process, the rapid operating speed of the transfer robot creates vibrations throughout the inline system. The magnitude of these vibrations is so great that respective elements which connect the units or which constitute the units may be loosened.
In particular, elements of the baking unit can come loose due to the vibrations generated by the transfer robot. The baking unit includes a chamber having a cover, and a hot plate disposed under the cover to heat a layer of photoresist on a wafer. More specifically, a wafer coated with photoresist is transferred into the baking unit by the transfer robot. Then, lift pins are raised through the hot plate to support the wafer held by the transfer robot whereupon the transfer robot is withdrawn from the chamber. Subsequently, the lift pins are lowered to lower the wafer onto the hot plate, and the baking process is carried out. However, the hot plate may be unbalanced in the case in which fasteners of the baking unit are loosened by vibrations generated by the transfer robot. That is, the upper surface of the hot plate may become skewed relative to the horizontal. As a result, the wafer may slide out of position on the hot plate as it is loaded onto the hot plate by the lift pins. Therefore, the wafer cannot provide heated uniformly and as such, the thickness of the layer of photoresist becomes non-uniform. Thus, some portions of the photoresist layer may not be developed properly during the developing process. If this occurs, the pattern of the developed photoresist layer may not have the desired CD (Critical Dimension). Furthermore, the wafer may even slide off of the hot plate as it is loaded onto the hot plate by the lift pins. In this case, the wafer may break on the bottom of the chamber. Thus, not only is the wafer destroyed but the baking unit is contaminated by particles of the broken wafer, thereby creating downtime in the photolithography equipment while the baking unit is cleaned.
Also, the cover of the baking unit may be dislocated by vibrations generated by the transfer robot. If the cover of the baking unit is not horizontal, heat can leak out from the chamber past the cover. As a result, temperature variations occur throughout the space in which the wafer is heated and thus, across the wafer. Therefore, the same problems as described above occur, i.e., the photoresist is not developed completely or the CD of the pattern does not correspond to the desired CD.
At present, the level of the hot plate or cover of the baking unit is checked by the naked eye. However, slight changes in the attitude of the hot plate or cover might not be discerned by the naked eye. Furthermore, even if changes in the attitude of the hot plate or cover are discerned, different technicians will level the hot plate or cover differently according to their own skill levels. Therefore, it is difficult to maintain uniformity in a baking process carried out in an inline system of photolithography equipment.